1. Field of the Invention
The present invention relates to a fuel injection control device of an internal combustion engine, and more particularly to a fuel injection control device which directly injects fuel into the inside of a combustion chamber of an engine while controlling a fuel pressure in a pressure storage chamber to a high-pressure target fuel pressure.
2. Description of the Related Art
Recently, an internal combustion engine which controls a fuel pressure in a pressure storage chamber such that the fuel pressure assumes an optimum high pressure value for a combustion state and directly injects fuel into a combustion chamber has been commercialized and one example of the constitution of a fuel supply system of this type of internal combustion engine is explained in conjunction with FIG. 4.
In FIG. 4, a high-pressure pump 20 is provided for pressurizing the fuel to a high pressure and the high-pressure pump 20 includes a cylinder 21, a plunger 22 which reciprocates in the inside of the cylinder 21, and a pressurizing chamber 23 which is defined and formed by an inner peripheral wall surface of the cylinder 21 and an upper end surface of the plunger 22. A lower end of the plunger 22 is brought into pressure contact with a cam 25 which is formed on a camshaft 24 of the engine, wherein due to the rotation of the cam 25 induced by the rotation of the camshaft 24, the plunger 22 reciprocates in the inside of the cylinder 21 thus changing a volume inside the pressurizing chamber 23.
Further, an inflow passage 30 which is connected to an upstream of the pressurizing chamber 23 is connected with a fuel tank 32 by way of a low pressure pump 31. Here, the low pressure pump 31 sucks and discharges the fuel in the fuel tank 32 and the fuel discharged from the low pressure pump 31 is regulated to a given low pressure value by a low-pressure regulator 33 and, thereafter, the fuel is introduced into the inside of the pressurizing chamber 23 by way of a check valve 34 when the plunger 22 descends in the inside of the cylinder 21.
On the other hand, a supply passage 35 which is connected to a downstream of the pressurizing chamber 23 is connected to a pressure storage chamber 50 by way of a check valve 36, wherein the pressure storage chamber 50 holds the high-pressure fuel discharged from the pressurizing chamber 23 and, at the same time, distributes the fuel into fuel injection valves 51. Further, the check valve 36 is provided for restricting the back flow of the fuel from the pressure storage chamber 50 to the pressurizing chamber 23.
Further, a relief valve 37 which is connected with the pressure storage chamber 50 is a normally-closed valve which is opened at a given valve-opening pressure or more. That is, when the fuel pressure in the inside of the pressure storage chamber 50 is elevated to the above-mentioned valve-opening pressure or more, the relief valve 37 is opened so that the fuel in the inside of the pressure storage chamber 50 is made to return to the fuel tank 32 through a relief passage 38 and hence, the excessive increase of the fuel pressure in the inside of the pressure storage chamber 50 is prevented.
A discharge flow rate control valve 10 provided between a supply passage 35 and a spill passage 39 is, for example, a normally-open electromagnetic valve. During a period in which the plunger 22 is moved upwardly in the inside of the cylinder 21, so long as a valve-opening control of the discharge flow rate control valve 10 is performed, the fuel which is discharged from the pressurizing chamber 23 to the supply passage 35 is made to return from the spill passage 39 to the inflow passage 30 so that the high-pressure fuel is not supplied to the pressure storage chamber 50. Then, after the discharge flow rate control valve 10 is closed at a given timing during the upward movement of the plunger 22 in the inside of the cylinder 21, the pressurized fuel discharged from the pressurizing chamber 23 to the supply passage 35 is supplied to the pressure storage chamber 50 through the check valve 36.
To an ECU 60 which constitutes an electronic control unit, detection signals from a rotational speed sensor 62 which detects a rotational speed of an engine 40, an accelerator position sensor 64 which detects a step-in amount of an accelerator pedal 63 and the like are inputted. The ECU 60 determines a target fuel pressure PO based on these engine operation information, and performs a feedback control of open/close timing of the discharge flow rate control valve 10 such that a fuel pressure PR detected by a fuel pressure sensor 61 which detects the fuel pressure in the inside of the pressure storage chamber 50 agrees with the target fuel pressure PO.
Further, the ECU 60 calculates a basic fuel injection flow rate which makes an air-fuel ratio detected by an air-fuel ratio sensor 66 arranged on an exhaust pipe assume a target air-fuel ratio based on an intake air flow rate detected by an air flow sensor 65, an engine rotational speed detected by the rotational speed sensor 62, the fuel pressure in the inside of the pressure storage chamber 50 detected by the fuel pressure sensor 61 and performs a drive control of the fuel injection valves 51.
Next, one example of the inner structure of the discharge flow rate control valve 10 is explained in conjunction with FIG. 5A and FIG. 5B.
To one end of a spill valve plunger 11, a spill valve 12 which is interlocked with the spill valve plunger 11 is connected, while to another end of the spill valve plunger 11, a spring 13 is connected. When a solenoid 14 is not energized, the spill valve 12 which is interlocked with the spill plunger 11 is pushed downwardly by a spring force of the spring 13 thus providing an valve opening state in which a supply passage 35 and a spill passage 39 are communicated with each other (FIG. 5A).
On the other hand, when the solenoid 14 is energized by the ECU 60, an electromagnetic force which the solenoid 14 generates overcomes the spring force of the spring 13 and attracts the spill valve plunger 11 upwardly. As a result, the spill valve 12 which is interlocked with the spill valve plunger 11 is also pulled upwardly thus providing an valve closed state in which the supply passage 35 and the spill passage 39 are—interrupted from each other (FIG. 5B).
Next, in conjunction with FIG. 6, the relationship between the manner of operation of the discharge flow rate control valve 10 and a fuel amount which is supplied from a high-pressure pump 20 to a pressure storage chamber 50 is explained.
A plunger 22 of the high-pressure pump 20 repeats the upward and downward movements between a minimum lift position and a maximum lift position in an interlocking manner with the rotation of a cam 25 of the engine 40. Then, as mentioned above, in a fuel intake stroke in which the plunger 22 descends from the maximum lift position to the minimum lift position, fuel is sucked into the inside of a pressurizing chamber 23 of the high-pressure pump 20 from an intake passage 30.
In a fuel discharge stroke in which the plunger 22 ascends from the minimum lift position to the maximum lift position, when the solenoid 14 is not energized, the discharge flow rate control valve 10 assumes a valve opening state and hence, fuel discharged from the high-pressure pump 20 is made to return to the intake passage 30 from the supply passage 35 through the spill passage 39 and the fuel is not supplied to the pressure storage chamber 50. Further, when the solenoid 14 is energized at given timing, the discharge flow rate control valve 10 assumes a valve closed state and hence, the supply passage 35 and the spill passage 39 are interrupted from each other, and the fuel which is discharged to the supply passage 35 from the pressurizing chamber 23 is supplied to the pressure storage chamber 50 during a period that the plunger 22 moves upwardly thereafter.
Due to the above-mentioned operations, to supply a portion of the fuel which the high-pressure pump 20 discharges to the pressure storage chamber 50, as indicated by a period Ta for [1] partial discharge control shown in FIG. 6, the solenoid 14 is energized from the middle portion of the fuel discharge stroke. Then, only the fuel (hatched portion A) discharged to the supply passage 35 from the pressurizing chamber 23 during the period Ta in which the solenoid 14 is energized is supplied to the pressure storage chamber 50.
Further, to supply the whole fuel which the high-pressure pump 20 discharges to the pressure storage chamber 50, as indicated by a period Tb for [2] 100% discharge control shown in FIG. 6, the solenoid 14 is energized from the beginning of the fuel discharge stroke. Then, the fuel (hatched portion B) discharged to the supply passage 35 from the pressurizing chamber 23 during the period Tb in which the solenoid 14 is energized is supplied to the pressure storage chamber 50. That is, the maximum amount of fuel which can be discharged by the high-pressure pump 20 is supplied to the pressure storage chamber 50.
To make the fuel supplied to the pressure storage chamber. 50 zero, as indicated by a period for [3] 0% discharge control (however, NE<Nm) shown in FIG. 6, the solenoid 14 is not energized from the beginning to the end of the fuel discharge stroke. Then, the whole fuel discharged from the high-pressure pump 20 is made to return to the inflow passage 30 through the spill passage 39 and hence, the fuel is not supplied to the pressure storage chamber 50.
Next, the discharge flow rate characteristic of the high-pressure pump 20 is explained in conjunction with FIG. 7.
In FIG. 7, the engine rotational speed NE is taken on an axis of abscissas and in case the high-pressure pump 20 is driven in an interlocking manner with a camshaft 24 of the engine 40, usually, the rotational speed NP of the high-pressure pump 20 and the engine rotational speed NE have the relationship NP=NE÷2.
Further, the fuel discharge flow rate QP of the high-pressure pump 20 is taken on an axis of ordinates and the maximum discharge flow rate which can be discharged by the high-pressure pump 20 with respect to the engine rotational speed NE becomes the flow rate at the time of 100% discharge control indicated by a chain line in FIG. 7.
Although the minimum discharge flow rate of the high-pressure pump 20 with respect to the engine rotational speed NE is, as indicated by a solid line in FIG. 7, designed to assume zero irrespective of the engine rotational speed NE, in an actual operation, there may be a case in which the minimum discharge flow rate assumes a flow rate at the time of 0% discharge control indicated by a broken line in FIG. 7.
That is, the minimum discharge flow rate when the engine rotational speed NE is Nm or less can be controlled to zero as designed. However, when the engine rotational speed NE falls in a high speed rotation region of Nm or more, the minimum discharge flow rate is increased larger than zero. For example, when the engine rotational speed NE is Nn (>Nm), the minimum discharge flow rate QP=qn is discharged at minimum. The cause of such a phenomenon is explained hereinafter.
To make the fuel supplied to the pressure storage chamber 50 zero or null, as mentioned previously, the solenoid 14 is not energized from the beginning to the end of the fuel discharge step and the spill valve 12 is in a state that the spill valve 12 is pushed downwardly by the spring force of the spring 13 (FIG. 5A).
Here, the fuel which is discharged into the supply passage 35 from the pressuring chamber 23 flows into the spill passage 39 through the spill valve 12 in the valve opening state, wherein along with the increase of the engine rotational speed NE, the flow speed of the fuel which passes the spill valve 12 is also increased and the maximum pressure which is generated in the inside of the supply passage 35 is gradually increased.
When the maximum pressure in the inside of the supply passage 35 becomes excessively high, a portion of the fuel which the high-pressure pump 20 discharges does not flow into the spill passage 39 and flows out to the pressure storage chamber 50 side. Further, in a worst case, the pressure in the inside of the supply passage 35 overcomes the spring force of the spring 13 which pushes down the spill valve 12 and hence, the spill valve 12 is pushed up whereby, even when the solenoid 14 is not energized, the discharge flow rate control valve 10 assumes the valve closed state. In this manner, when the discharge flow rate control valve 10 is automatically closed in spite of the fact that the solenoid 14 is not energized, as indicated by the period of [4]0% discharge control (here, NE≧Nm) shown in FIG. 6, even when the solenoid 14 is not energized, there is a possibility that the discharged fuel (a hatched portion C in FIG. 6) in the automatically closed period of the discharge flow rate control valve 10 is undesirably supplied to the pressure storage chamber 50.
As a countermeasure to overcome the above-mentioned drawbacks, it may be possible to enlarge a fuel passage area of the spill valve 12 so as to reduce the maximum pressure which is generated in the supply passage 35. However, since this countermeasure requires the remodeling of the discharge flow rate control valve 10, the manufacturing cost is pushed up. Further, it may be also possible to increase the spring force of the spring 13 so as to prevent the automatic closing of the spill valve 12 of the discharge flow rate control valve 10. However, as a drawback of such a countermeasure, the valve-closing response property of the spill valve 12 at the time of performing the normal control is lowered and there exists a possibility that the fuel pressure controllability is worsened. Further, even when the above-mentioned countermeasures are put into practice, it is considered that similar drawbacks will arise when impurities contained in the fuel are stacked on a periphery of the spill valve 12 so that a passage area is narrowed or the spring force of the spring 13 is lowered along with the lapse of time.
The influence which the above-mentioned drawbacks affect the engine is explained in conjunction with the time chart shown in FIG. 8.
FIG. 8 shows the change of various state variables when an accelerator is made to return by a given amount from a state in which the high-load steady state operation (fuel injection flow rate=qf) is performed with the engine rotational speed NE=Nn (>Nm).
Up to a point of time t1 in FIG. 8, a fixed intake air flow rate qa1 which corresponds to a step-in amount apl (a fixed value) of the accelerator pedal 63 is sucked by the engine, while the fuel injection flow rate qf indicated by a solid line which corresponds to the intake air flow rate qa1 is injected from the fuel injection valves 51 and the steady state operation is performed with the engine rotational speed NE=Nn. Here, the pump discharge flow rate which is equal to the fuel injection flow rate qf is discharged by the high-pressure pump 20 and is supplied to the pressure storage chamber 50 so that the fuel pressure PR in the inside of the pressure storage chamber 50 agrees with the target fuel pressure PO.
When the step-in amount of the accelerator pedal 63 is made to return from ap1 to ap2 (<ap1) at the point of time t1, corresponding to the decrease of the intake air flow rate from qa1, the fuel injection flow rate qf is also lowered. As a result, the generated torque of the engine is lowered and the engine rotational speed NE is also gradually lowered. However, compared to the lowering speed of the intake air flow rate, the lowering speed of the engine rotational speed NE is slow due to the inertia of motion of the engine.
When the operation passes the point of time t2, corresponding to the decrease of the intake air flow rate, the fuel injection flow rate is lowered to a value equal to qn or below. Here, although the engine rotational speed NE is slightly lowered, since the engine rotational speed NE is held to the rotational speed which is substantially close to Nn, the discharge flow rate of the high-pressure pump 20 indicated by a broken line is not lowered to approximately qn which is the minimum discharge flow rate when the engine rotational speed NE is approximately Nn. As a result, the fuel injection flow rate becomes smaller than the discharge flow rate of the high-pressure pump 20 so that the fuel pressure PR in the inside of the pressure storage chamber 50 starts elevation against the target fuel pressure PO. Here, the reason that the fuel pressure PR in the pressure storage chamber 50 is elevated is that the fuel discharge flow rate of the high-pressure pump 20 which supplies the fuel to the inside of the pressure storage chamber 50 becomes larger than the fuel injection flow rate which consumes the fuel in the inside of the pressure storage chamber 50 and hence, the fuel charge amount in the inside of the pressure storage chamber 50 is increased.
When the operation reaches the point of time t3, due to the lowering of the engine rotational speed NE, the minimum discharge flow rate of the high-pressure pump 20 becomes gradually lower than the fuel injection flow rate and hence, the increase of the fuel in the inside of the pressure storage chamber 50 is stopped. Then, after the lapse of the point of time t3, it is possible to perform the control such that the minimum discharge flow rate of the high-pressure pump 20 becomes smaller than the fuel injection flow rate and hence, once the fuel amount in the inside of the pressure storage chamber 50 is started to be decreased, the fuel pressure PR is also started to be decreased. Here, after the lapse of the point of time t4, since the engine rotational speed NE becomes NE<Nm, it is possible to control the minimum discharge flow rate of the high-pressure pump 20 to zero and hence, the fuel pressure PR in the inside of the pressure storage chamber 50 is lowered to the target fuel pressure PO.
In this manner, in a state that the discharge flow rate of the high-pressure pump 20 becomes larger than the fuel injection flow rate and hence, the fuel pressure PR is elevated and does not agree with the target fuel pressure PO, the combustion state which is optimum for the engine cannot be obtained whereby the exhaust gas is deteriorated or the fuel pressure PR becomes excessively high whereby the fuel injection valves 51 cannot be driven with the given response property and, in a worst case, there exists the fear of the occurrence of the engine stop.
To overcome the above-mentioned fear, there exists a technique which is proposed in JP-A-2000-303883 (hereinafter referred to as patent literature 1).
In this patent literature 1, in place of the relief valve 37 explained in FIG. 4, an electromagnetic pressure release valve whose open-close operation is controlled by an ECU 60 is adopted, wherein when the fuel pressure PR is to be lowered, the valve opening control of an electromagnetic pressure release valve is performed. However, such a conventional device requires a control system for the electromagnetic pressure release valve and hence, the manufacturing cost is pushed up.